1. Field of the Invention
The present invention relates to a plasma etching technology suitable for use in manufacturing a semiconductor device, and particularly to a method for stably starting a discharge upon plasma etching using helium (He) gas.
2. Description of the Related Art
With an improvement in performance of a semiconductor integrated circuit and high integration thereof, a plasma etching technology is required to provide a high accuracy dimension and high etching selectivity for an underlying film. The dimensions of a gate electrode of a MOS transistor at micro-fabrication become important factors for determining a threshold voltage of an elemental device. Gate electrode dimensions of 0.1 μm-level have been required together with miniaturization. Although the current leading-edge semiconductor exposure is one using ArF scanner light, an exposure system is expensive, the life of an optical lens is short and running costs are high. Therefore, developments have been put forward in the direction to further reduce critical dimensions at KrF scanner exposure which has heretofore been used.
However, a practical minimum pattern size is about 0.16 μm at the KrF scanner exposure. It is difficult to form a fine pattern of 0.1 μm level under the present circumstances. As a method for overcoming the above exposure problem, there is known a method for reducing dimensions by etching rather than reducing exposure dimensions. As a general method, may be mentioned, for example, a slimming technique for anisotropically etching a resist mask by means of a mixed gas of helium (He) and oxygen (O2) to thereby reduce or scale down it.
The plasma etching technology using the mixed gas features that no reactivity occurs in many insulating materials such as silicon nitride (SiN), silicon oxide (SiO2), etc. used in a semiconductor process in general and even in conductive materials such as polysilicon (poly-Si), tungsten silicide (WSi), titanium nitride (TiN), etc., and hence no etching is effected thereon. Therefore, this gas is most suitable as an etching gas for slimming a resist and generally used in the slimming technique.
However, plasma etching in which O2 gas is added with He gas as a principal component, involves a problem that a discharge is hard to take place in an etching chamber. The electron disposition of He atoms makes a closed shell and first ionization energy is maximum like 24.5 eV in all atoms. Therefore, the ignition of a low-voltage glow discharge using He gas is hard to take place. It was thus difficult to stably start the discharge.